Edit ScummVM.ini in the game installation directory and edit the lines
speech_volume=192
sfx_volume=192
music_volume=192
I believe the maximum value is 255.
You will have to restart the game to hear the effect.

You could also try changing this line (this seems to be a percentage, so change it to 50, for example):
midi_gain=100

By the way, I recommend to add the following line to the [t7g] section after the "platform=pc" line for better music quality:
music_driver=windows_Microsoft GS Wavetable Synth

Edit: Important note: You may find, that you can't save your changes or they are immediately reverted. This usually happens when using Windows Vista/7 and installing the game in the "Program Files"/"Program Files (x86)" Directory.

So as a general rule I recommend to NOT install GOG games (or any older games that you want to improve with ini-tinkering for that matter) in the "Program Files"/"Program Files (x86)" directories.

Thanks for your quick response, but that doesn't quite work.

The characters' speech (with the actors) indeed became louder, but the voice explaining the mini games is still inaudible and the music volume doesn't change whether I set it to 0 or 255.
Changing midi_gain or adding music_driver=windows_Microsoft GS Wavetable Synth doesn't do anything either.

speech_volume=192
sfx_volume=192
music_volume=125
midi_gain=50

seems to have helped me

Thanks that really helped

No worries :D

Thank goodness! I was about to go deaf. :-)

BTW, I took the ownership of that GOG folder and was able to edit the ini file.

Edit: Wrote too soon. Music is still too loud. Will investigate.

Edit 2: It works when I start the game as admin. Music volume now tolerable.

I had to set mine like this:

music_volume=40
speech_volume=200
midi_gain=0
sfx_volume=192

Otherwise the music is still too loud.

Stupid question. But what if you're using Boxer and don't have a Scumm ini? I installed mine in boxer from an image of my copy of the original but I'm having this same problem. But I don't know what file to edit, because for some reason boxer didn't install the program files and dos files. It just has the game.

What about Mac users? There doesn't seem to be a ini file when I look.